1. Field of the Invention
The present invention relates to a technique for fabricating a semiconductor integrated circuit with a greater device density, and more particularly, to field-effect devices adapted for larger scales of integration and also to a method of fabricating such field-effect devices. A semiconductor device according to the invention is used especially advantageously as a nonvolatile semiconductor memory device having floating gates.
2. Description of the Related Art
Some conventional semiconductor devices are formed on a flat plane. Field-effect devices such as a MOS field-effect transistors (MOSFET) or a MISFET have a source, a drain, and a channel arranged substantially on a flat plane. The drain current is made to flow in a direction parallel to the substrate. In this planar device, however, limitations are imposed on the device area reduction as a matter of course. Therefore, in an attempt to obtain larger device densities, methods for manufacturing a planar device from plural layers and methods of fabricating a device structure itself in a form different from a planar structure have been discussed. An example of the latter methods includes a vertical-channel MOSFET proposed by us in Japanese Patent Unexamined Publication No. 13627/1994, which, in turn, corresponds to U.S. Pat. No. 5,350,937 and U.S. pending application Ser. No. 08/268,448, the disclosures of which are herein incorporated by reference. In particular, the drain is located over or under the source so that the drain current flows substantially vertically. This structure permits larger component densities.
The above-cited Japanese Patent Unexamined Publication No. 13627/1994 pertains to a nonvolatile semiconductor memory. Specifically, a floating gate and a control gate are formed by anisotropic etching on side surfaces of elevated portions which are formed on a semiconductor substrate. It is to be noted, however, only the fundamental device structure is shown. Neither the whole memory construction nor the fabrication process is described in detail. For example, with respect to its peripheral circuit, few mentions are made of its structure and fabrication process.